Landers residents awoke yesterday to find that during the night something like a flying saucer had landed in the demonstration garden at the Landers Post Office. The eight-foot-wide, silvery-black, spaceship-looking installation appears to be on plywood and embedded in concrete. No one from the post office, Landers  Association, or even the water district could say who put the piece there, or rather, when it had landed. There is a cutout in the center of the saucer, like the kind people put…